General and galvanic corrosion of magnesium coupled to graphite embedded in hydraulic binders corrosion rate, rust layer properties and binder mechanical strains
Résumé
Graphite and magnesium alloys wastes were generated during the reprocessing phase of spent fuel assemblies of the former nuclear reactors in France. Conditioning of these low to intermediate level wastes in cementitious materials is being addressed here. The study is held in the framework of a PhD thesis aiming to develop a numerical model able to predict the generation of stresses in such waste packages during their lifetime.In fact, magnesium is one of the most reactive metals with a standard normal potential of -2.37mV/SHE [1]. Once embedded in a hydraulic binder with alkaline pH and high internal humidity, oxidation reactions occur. The subsequent formation of corrosion products around the alloy may result in tensile stresses development in the surrounding binder that could lead to cracking risks. Thus, general and galvanic corrosion (when coupled with graphite) of the metal in the packages should be properly addressed.Hence, weight losses, electrochemical techniques and microscopic observations together with chemical analysis methods (Raman and Energy Dispersive X-ray Spectroscopies) are used to characterize the metal's corrosion in three binders. These latter consist of two different ordinary Portland cement and an alkali-activated blast furnace slag mortars [2]. Also, the mechanical properties of these products are determined using nano-indentation technique. The results prove that the use of alkali-activated slag is beneficial for the metal's galvanic corrosion while the general corrosion behavior is comparable in all studied mortars.Additionally, corrosion induced binders' strains are measured using a coupled experimental setup specially designed. Linear variable differential transformer (LVDT) sensors are used to monitor global strains of cementitious samples with normalized dimensions containing coupled magnesium and graphite electrodes. The potentiostatic electrochemical technique is applied in order to determine the evolution of the simultaneous corrosion current intensity in the sample.